The present invention relates to a method and apparatus for monitoring cracks of a rotatable body.
A rotatable body forming a part of a rotary machine can be cracked due to, for example, creep fatigue or stress corrosion with such cracks causing, for example, operational disturbances, machine breakdowns, or the like thereby present serious and dangerous situations for operating personnel, especially with turbine motors of thermal or nuclear power generation equipment. Consequently, it is imperative that measures should be taken to avoid in advance any accidents which may occur as a result of cracks in the rotatable body of the rotary machine and an effective repair of the rotatable body should be facilitated.
Conventionally, in order to monitor cracks, acoustic emission signals, hereinafter referred to as AE signals, generated by a cracking in a rotatable body such as a rotor have been detected; however, in the conventional methods, it is impossible to monitor the progress of cracking. While it has been proposed to detect vibration signals of the rotatable body and then to monitor cracks by utilizing the AE signals, a detection of cracks according to this proposal is restricted because the cracks can be detected only if the speed of the rotatable body is changed and if the size of the cracks exceeds a predetermined value.
An object of the present invention resides in providing a method and apparatus for detecting a cracking of a rotatable body and for monitoring the progress of such cracking.
This object if achieved according to the invention by a method for monitoring cracks of a rotatable body comprising the steps of detecting cracks by measuring accoustic emission signals, measuring vibrations of the rotatable body, calculating the position of cracks on the basis of the measured accoustic emission signals, comparing assumed vibrations with measured vibrations and evaluating the depth of cracks from the result of this comparison.
Hereby, it is possible to detect the beginning of cracking of a rotatable body very quickly and also to monitor accurately the progress of cracking on an on line basis because not only accoustic emission signals but also vibration signals are measured and the depth of cracks can be evaluated on the basis of the result of comparison of assumed vibrations and measured vibrations.
Preferably, the detecting of cracks by measuring accoustic emission signals is perfomred by detecting first accoustic emission signals from two first accoustic emission detectors attached to the rotatable body, detecting second accoustic emission signals from two second accoustic emission detectors attached to the bearings of the rotatable body and by eliminating accoustic emission signal portions caused by other reasons than cracking from those accoustic emission signals. According to this method, cracking can be detected accurately.
Other advantages and optional features of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, several embodiments in accordance with the present invention.